Among the changes in automotive technology in the 1980s has been the gradual displacement of the carburetor by fuel injectors. The traditional carburetor produced the fuel-and-air mixture needed for engine operation. In the carburetor, the fuel is dispersed in the form of tiny droplets in a stream of air. As a result of the heat absorbed on the way to the cylinder, these fuel droplets become vaporized, and the mixture thereby becomes an inflammable gas. The fuel-and-air mixture thus formed enters the combustion chamber of the cylinder, and is consumed on the power stroke.
In a fuel injected engine, fuel is injected directly into the combustion chamber of the cylinder. The injection nozzle or orifice must therefore be designed to atomize the fuel prior to its injection into the cylinder, so that complete combustion of the fuel can take place. As a fuel injected engine is operated, impurities contained in the fuel, such as varnish, dirt and other small particles, can build up in the nozzle or orifice that atomizes the fuel. When such build up occurs, the fuel injector can become partially plugged, and the fuel injector may occasionally fail to deliver the quantity of fuel required for combustion. As a result of the failed combustion due to lack of sufficient fuel, the cylinder misfires. Such an intermittent cylinder misfire or "lean-roll" condition causes the composition of the gases in the exhaust manifold to differ from those present when no misfire or lean-roll occurs. In particular, the amount of hydrocarbons (HC), carbon monoxide (CO) and oxygen (O.sub.2) in the exhaust manifold gas stream will change in comparison to when complete combustion occurs.
In vehicles equipped with catalytic converters, the detection of the lean-roll misfire condition is more complicated than for vehicles without such converters. When the catalytic converter is functioning properly, it oxidizes essentially all of the HC and CO exiting the cylinders to carbon dioxide (CO.sub.2) and water vapor. The concentrations of HC and CO in the exhaust gas stream exiting the catalytic converter (and also the tailpipe) are so low that they cannot be measured accurately with existing HC/CO analyzers. Likewise, during intermittent misfire or lean-roll resulting from, for example, a partially plugged port fuel injector, HC and CO will not appear in the exhaust gases exiting the catalytic converter, and conventional HC/CO analyzers lose their value as diagnostic tools for locating such partially plugged port fuel injectors.
The lean-roll phenomenon associated with fuel injector plugging can best be understood with reference to FIG. 1. In a typical engine 10 having cylinders 12a-f, port fuel injectors 14a-f, respectively, are connected to their corresponding cylinders. Fuel is fed to fuel injectors 14a-f by fuel line 16, the open end of which in FIG. 1 is connected to the fuel pump (not shown). Air is fed to fuel injectors 14a-f by air line 15, the open end of which is connected to the air filter (not shown). Exhaust gas stream 18 exiting cylinders 12a-f is carried to catalytic converter 20 in exhaust manifold 26. Exhaust gas stream 24 exiting catalytic converter 20 is released from the vehicle to the atmosphere by tailpipe 22.
As stated earlier, the partial plugging of any of fuel injectors 14a-f will cause its corresponding cylinder 12a-f to intermittently misfire. This is called the lean-roll misfire condition. Lean-roll can occur when cylinders 12a-f misfire individually or simultaneously, as partial plugging may exist in one or more of fuel injectors 14a-f. The present invention provides a method by which such partially plugged port fuel injectors can be located. Once located, the partially plugged fuel injectors can be cleaned or replaced, and the lean-roll misfire condition thereby eliminated.
In the preferred embodiment of the present invention, the apparatus used to detect the lean-roll condition is the MIXWHIZ.TM. MISFIRE MONITOR sold by Blanke Industries Inc., P.O. Box 1178, Barrington, Ill. 60010. The operation of the MIXWHIZ.TM. MISFIRE MONITOR is described in U.S. Pat. No. 4,030,349 entitled "IMPROVED ENGINE ANALYSIS APPARATUS" filed in the name of John D. Blanke et al. and U.S. Pat. No. 4,031,747 entitled MISFIRE MONITOR FOR ENGINE ANALYSIS HAVING AUTOMATIC RESCALING filed in the name of John D. Blanke, both patents of which are incorporated herein by reference in their entireties. In the Blanke et al. '349 patent, a misfire monitor is described which is capable of detecting lean-roll in an internal combustion engine by comparing the rate of change of oxygen in the exhaust gases with respect to time (dO/dt) against a limit. In the Blanke '747 patent, a misfire monitor is described which is capable of automatically adjusting the parameters used in the testing for a lean-roll condition to a standard range whereby the misfire monitor can be used on various engine types with little or no operator intervention.
It has been found that an oxygen analyzer like the MIXWHIZ.TM. MISFIRE MONITOR is more suitable than an HC/CO analyzer for detecting intermittent misfires or lean-roll because the concentration of uncombusted oxygen, unlike uncombusted HC and CO, is not affected by the catalytic converter. Thus, oxygen is available for analysis at the tailpipe whether or not the vehicle is equipped with a catalytic converter. As described in the '349 and '747 patents, the MIXWHIZ.TM. MISFIRE MONITOR is a very fast oxygen analyzer which employs a sensor and associated electronics to detect intermittent changes in oxygen concentration. When a misfire occurs, the oxygen in the exhaust gas stream quickly increases and then decreases to a steady state oxygen level. This spike of oxygen in the exhaust gas stream lasts only for a fraction of a second. The oxygen spike produced by the misfire triggers the sensor and electronic circuitry of the MIXWHIZ.TM. MISFIRE MONITOR to actuate an alarm. Another major circuit of the misfire monitor converts the oxygen sensor output into a direct readout of oxygen in the tailpipe, thus allowing precise adjustment of fuel-to-air ratios.
In the case of a partially plugged port fuel injector, the plugging causes the cylinder to fail to ignite because of inadequate fuel or leanness. As a result of the misfire, the slug of oxygen from the unignited cylinder causes the exhaust oxygen to rapidly increase and then come back down when the cylinder associated with the partially plugged fuel injector fires on the next power stroke. The MIXWHIZ.TM. MISFIRE MONITOR detects this misfire and any other bursts of oxygen caused by later misfires. Whenever the MIXWHIZ.TM. MISFIRE MONITOR detects an oxygen spike, it signals an alarm by, for example, the beep of a horn, to let the user know that a misfire has occurred.
The present invention is directed to the difficulties inherent in locating partially plugged port fuel injectors in internal combustion engines. As stated earlier, the existing HC/CO analyzers are ill-suited for locating such partially plugged port fuel injectors because of their inability to detect a lean-roll condition in view of the near-complete consumption of HC and CO by the catalytic converter. Moreover, the apparatus and methods described in the '349 and '747 patents are directed to carburetor-based engines and do not describe or suggest the use of a misfire monitor or oxygen analyzer with fuel injected engines, much less to locate partially plugged fuel injectors.